This disclosure relates generally to methods and apparatus for acquiring and analyzing cores from subterranean formations. More particularly, this disclosure relates to methods and apparatus for utilizing an absorbent core barrel assembly to retain fluids that are ejected from a core and methods of analyzing the core and retained fluids.
Formation coring is a well-known process for obtaining a sample of a subterranean formation for analysis. In conventional coring operations, a specialized drilling assembly is used to obtain a cylindrical sample of material, or “core,” from the formation and retain that core within a core barrel so that the core can be brought to the surface. Once at the surface, the core can be analyzed to reveal formation data such as permeability, porosity, and other formation properties that provide information as to the type of formation being drilled and/or the types of fluids contained within the formation.
In many hydrocarbon-bearing formations, the hydrocarbons are entrained within the formation at high pressures. As a core is being retrieved to the surface, the pressure acting on the core can be reduced and gas entrained in the core can expand and migrate out of the core. The expanding gases can also push formation fluids out of the core. In conventional coring operations, the formation fluids and gases are often lost as the core is retrieved to the surface, thus limiting the analysis that can be performed.
One method used to counteract the loss of formation fluids is “sponge coring.” Sponge coring is similar to conventional coring but the coring assembly includes a core barrel that has an annular sponge that surrounds the core as it is acquired. The annular sponge can absorb formation fluid that is expelled from the core and can hold the fluid as the sample is retrieved to the surface. At the surface, the absorbed fluids can be analyzed to provide additional information about formation properties or formation fluids.
In conventional sponge coring tools, the sponge material is molded directly into a core barrel, or into a liner that fits into the core barrel. In many applications, an annular mold is formed by placing a cylindrical mandrel, which has a diameter substantially equal to the core to be acquired, inside a cylindrical liner. A liquid material (such as polyurethane), catalyst, and foaming agent are deposited into the mold and react to form a sponge material that fills the mold and hardens. During the molding process, the sponge material adheres to the liner or barrel and forms a non-adhering “skin” on the surface that contacts the mandrel. The mandrel is removed to leave an annular sponge adhered to the liner and having a circular hole through its center having the same diameter as the mandrel. The presence of the skin on the inner surface of the annular sponge limits absorption of fluid into the sponge and therefore requires a separate machining process to remove the skin and provide the necessary internal diameter to accept the core. Consistently and reliably machining the sponge material to the necessary diameter has proven to be a difficult process.
Conventional sponge coring tools are also susceptible to damage as the core moves through the annular sponge. In order to properly capture the formation fluid, the annular sponge is machined to an inner diameter that is closely matched to, or even in an interference fit with, the core that is being drilled. As the core moves relative to the annular sponge, the close engagement between the annular sponge and the core can result in the sponge being damaged. Once the sponge is damaged, it can interfere with the acquisition of the core or may lose the ability to effectively absorb fluids from the core and may therefore compromise the analysis sought to be performed. Attempts have been made to reinforce the annular sponge through strengthening members molded into the sponge material or by incorporating a non-absorbent retention mesh into the sponge material, but instances of damage to the annular sponge still occur.
The materials and methods used to form conventional sponge coring tools can also create limitations in the use of the technology. For example, the material used to form the annular sponge, often polyurethane foam, can interfere with some analysis, such as determining oil fluorescence using ultraviolet light. Further, conventional annular sponge material also tends to have a non-homogenous cross-section where permeability and absorbability of the material changes through the thickness of the material.
Thus, there is a continuing need in the art for methods and apparatus for acquiring and analyzing cores that overcome these and other limitations of the prior art.